A vehicle suspension apparatus supporting a sprung member such as a vehicle body comprises a spring and a damper, which are interposed/disposed between the sprung member and an unsprung member. A damping force control apparatus for variably controlling a damping force characteristic of the damper is known. Such a damping force control apparatus variably controls the damping force characteristic of the damper in response to a vibration state/condition of the sprung member on the basis of, for example, a skyhook control theory or a Nonlinear H .INF. (H∞) control theory.
Japanese Patent Application Laid-Open (kokai) No. Hei 6-247117 discloses a suspension apparatus comprising a damper having a structure configured in such a manner that, when either one of “a damping force characteristic when the damper is expanded/uncompressed” or “a damping force characteristic when the damper is compressed” is variably controlled, the rest of those is fixed to (or kept unchanged at) a low damping force characteristic. Damping force control means for controlling the damping force characteristic of the damper of this suspension apparatus variably controls the damping force characteristic of the damper as follows. The damping force characteristic is variably controlled in such a manner that the damping force characteristic is proportional to a value obtained by dividing a speed of a sprung-member in a vertical direction (a sprung-member-speed) by a sprung-member-unsprung-member-relative-speed (a vibration speed of the suspension apparatus), which is a relative speed between the sprung-member-speed and a speed of the unsprung-member in the vertical direction, when a product of the sprung-member-speed and the sprung-member-unsprung-member-relative-speed is positive. Further, the damping force characteristic of the damper is variably controlled in such a manner that the damping force characteristic is proportional to the sprung-member-speed, when the product of the sprung-member-speed and the sprung-member-unsprung-member-relative-speed is negative.